I would like to start a dark frame library but don't know few things ...If I take darks at -10 Celcius ,How much degree higher and lower ,+/- ,can I use those ???Is a master dark as good as putting all the darks in DSS ???Can I make my darks during cloudy nights using only the camera and cap on ,or do I have to fit it to the scope ???Thank you all for answering !Maxx

You can (and you should) take darks during cloudy nights. The camera cap is all you need.

You don't have to fit your camera back on the scope. A dark is just that, a picture with no light photons reaching the sensor so lens/scope does not matter. But duration and ISO do matter so make sure you match those to your light images.

I'd just be careful not to lay the camera on its back while taking darks. I have no data to prove this, but I'd be a little concerned that changing the heat losses through the back of the camera could have an effect on the sensor temperature. +/-3C is good up to about 25-30C (sensor temperature), between 30C and 40C +/-2C is better, and above 40C (yes I've imaged with a sensor temperature above 40C) the temperature needs to be pretty well matched.

Dude! imaging at 40 Celcius...40 C at night ? during the day it must have been terribly hot !You have a good point there ,when I image ,my camera's back touches nothing and temperature exits differently depending the angle of the camera.Thank you for helping me !Maxx

Looking at the noise v. temp charts for the chips, there's a difference at even 1 degree C. A degree or two probably isn't too bad, though - and I know that with an unregulated camera that much error is inevitable even within a given session. You can check - take two darks at different temperatures and subtract the cooler one from the other. What remains is the noise that the warmer dark won't correct at the lower temp because of that much temp error. Reliable darks is just one more reason for selecting a dedicated astro camera.

When I used a DSLR I relied more on handling the noise during combination. If you dither the frames via the autoguider, the noise will move around relative to objects in the image and a median combine will deal with whatever the darks miss.

I have been using Darks taken in August for images shot in January. I take Dark Flats and Bias frames at light temps, seems to work for me. What does a more seasoned APer feel about this thought?

That is fine for most, but consider adding a few new darks and retiring a few older darks from your library every now and then. This will keep you library up to date with your camera sensor changing over time.

I thought that dark frames lasted for the time I used the same camera...How long can I use a master dark then ?

I've heard it suggested you replace them every 6 months, but I've never seen anyone back that up with an actual study. One thing I wanted to mention is that it's a good idea to use a pause between frames to let the DSLR's chip cool between shots. When I was using a DSLR I discovered that the chip heats up significantly if you take many exposures in a row which effectively changes the temperature you are targeting. I can't remember if this happened with my 350 or 450, but I'll bet a lot of cameras will have this issue.

My educated guess is that 5 seconds would be enough, but I'm afraid I didn't test- shortly after I discovered this issue, I bought my ST8300 and rarely do DSLR imaging any longer. I'm sure 5 seconds would be much better than none.

I've been using uncooled cameras for the past 9 years and handling the darks is always a mixed bag. My CCD does automatic dark matching in real-time which is nice, but even there I update the libraries at least once a year. On the DLSR side I switched to using the internal dark subtraction. I used it last night with 600 second subs and my source images looked great! I haven't taken a stand alone dark frame with my DSLR in over a year. If you want to see something interesting look at the sensor temperature as a function of time as you take your darks. Also, look at each individual frame. As time goes on the sensor keeps getting warmer and the darks noisier.

40C sensor temperature,of course ,I knew that... I din't know that it was possible to see at what temperature my sensor is...I have a Canon T3 ,I hope I have that function ,I will look to find it !Thanks !Maxx

How do you check the sensor temp in your DSLR? Is that an option on some cameras?

John,

The temperature is stored in every .cr2 files.

You can use exiftool to extract the data... or you can use software to automatically add the sensor temperature in your image .cr2 file name at acquisition time if you're using a laptop to acquire your images. I wonder who makes such software